The present invention relates to a transmission for use on a vehicle of the primarily off-highway type, and more particularly, to such a transmission which includes both a hydrostatic transmission portion and a mechanical transmission portion.
The present invention could be used on a variety of vehicle types, including vehicles which travel, at least part of the time, as "on-highway" vehicles, but it is especially adapted for use in "off-highway" vehicles and will be described in connection therewith.
It has been generally well known to those skilled in the art to provide a transmission including a hydrostatic transmission portion and a mechanical transmission portion. A typical hydrostatic transmission comprises a variable-displacement pump hydraulically coupled to a motor (typically, of fixed displacement), and appropriate controls for varying the displacement of the pump. Although various types of pumps and motors can be utilized, it is preferable to use an axial piston pump and motor, wherein displacement of the pump is varied by changing the tilt angle of a tiltable swashplate, in a manner which is very well known to those skilled in the art.
The "mechanical transmission" may comprise a simple two-speed, shiftable, gear-type transmission, which effectively serves as a "range" selector for the hydrostatic transmission, to extend the speed range and gradability of the vehicle, while permitting the components of the hydrostatic transmission to be of a reasonable, and more economical size. Those skilled in the art will understand that the invention is not limited to use with a two-speed transmission, and a three-speed or perhaps even a four-speed transmission could be used, but for ease of illustration and explanation, the invention will be described in connection with a two-speed transmission.
Although various means of shifting the mechanical transmission may be provided, and the invention is not limited to any particular such means, the invention will be described in connection with a mechanical transmission which is provided with a shift cylinder having a neutral position, in which the mechanical transmission is disengaged and then, preferably on either side of neutral, a pair of positions corresponding to low gear and high gear.
In describing the system of the present invention, in order to avoid the confusion usually associated with references to "low" gear and "high" gear, the two gears in the mechanical transmission will be referred to simply as "first gear" (or Gear 1) and "second gear" (or Gear 2). In general, Gear 1 and Gear 2 may be thought of as corresponding to a first gear and a second gear in an automotive type transmission, wherein the ratio of engine speed to drive shaft speed is greater in first gear than in second gear, or stated another way, for a constant engine speed, vehicle speed will increase upon shifting from first gear to second gear. However, those skilled in the art will understand that references hereinafter to low and high gears, or Gear 1 and Gear 2, are by way of example and explanation, and not by way of limitation, except where the context clearly indicates otherwise, and references in the appended claims to first and second gear ratios are similarly not meant to be limiting. In other words, a reference in the appended claims to a "first gear ratio" can mean either the low gear or the high gear, and the term "first" is used simply to indicate that it is the first reference to a gear ratio.
Although a transmission of the type described, including both a hydrostatic portion and a mechanical portion, has been generally satisfactory in achieving the overall objectives noted above, one major disadvantage which has been present in the prior art transmissions has been the necessity to stop the vehicle in order to change the gear ratio (or "range") of the mechanical transmission. The necessity to stop the vehicle obviously results in greater operator effort and fatigue, but also decreases the amount of useful work which can be accomplished with the vehicle. Furthermore, if the vehicle is stopped on a grade (either uphill or downhill), such that the mechanical transmission has torque applied to it, the vehicle operator may have great difficulty performing the desired gear shift, or range selection, of the mechanical transmission.
Accordingly, it is an object of the present invention to provide an improved transmission including both a hydrostatic transmission portion and a mechanical transmission portion in which the shifting of the mechanical transmission may be performed "on the go," i.e., without stopping the vehicle, and preferably, with no immediate, discernible change of vehicle speed.
In order to accomplish the above-stated object, it is desirable to provide a hydrostatic transmission which is electronically controlled, i.e., one in which there is overall electronic control of the transmission, whereby displacement of the hydrostatic pump is coordinated with the shifting of the mechanical transmission to achieve the shift-on-the-go capability.
Unfortunately, simply combining an electronically controlled hydrostatic transmission and mechanical transmission, and coordinating the shifting of the two transmissions electronically, would frequently have the result that the shifting of the mechanical transmission would occur under conditions where such a shift is not desirable (or "feasible"). By way of example only, if the vehicle were loaded to such an extent that the vehicle would come to a stop during the shift operation, then the shift operation, under those particular conditions, would not be considered "feasible", and should not be allowed by the control system.
Accordingly, it is another object of the present invention to provide a transmission and control system of the type described above, which has the shift-on-the-go capability, but wherein the control system does not permit the occurrence of any shift which is not considered feasible, as that term will be described further hereinafter.
One important aspect of the type of system described above is to determine that a particular shift is feasible, and then complete the shift as quickly as possible (i.e., before the shift in question becomes "not feasible"). For example, in the condition described above, if there is concern about the loading on the vehicle bringing it to a stop before the shift is completed, then clearly, the quicker the shift is performed, the greater will be the chance of that particular shift being considered feasible, and being successfully performed. As those skilled in the art understand, the more quickly the shift can be completed, the greater the number of vehicles and applications which can utilize the transmission system of the present invention.
Accordingly, it is another object of the present invention to provide a transmission and control system having the ability to minimize the time required to achieve the various shifting operations by utilizing predictive control logic.